There is no CPSR bit for endianness in ARMv4 (ARM7TDMI) or ARMv5 (ARM9), so you need to use other means.
If your core implements system coprocessor 15, then you can check the bit 7 of the register 1:
MRC p15, 0, r0, c1, c0 ; CP15 register 1
TST r0, #0x80 ; check bit 7 (B)
BNE big_endian
B little_endian
However, the doc (ARM DDI 0100E) seems to hint that this bit is only valid for systems where the endianness is configurable at runtime. If it's set by the pin, the bit may be wrong. And, of course, on most(all?) ARM7 cores, the CP15 is not present.
There is a platform-independent way of checking the endianness which does not require any hardware bits. It goes something like this:
LDR R0, checkbytes
CMP R0, 0x12345678
BE big_endian
BNE little_endian
checkbytes
DB 0x12, 0x34, 0x56, 0x78
Depending on the current endianness, the load will produce either 0x12345678 or 0x78563412.